Conventional procedure for treating cupriferous ores and concentrates involves the steps of smelting the concentrate to produce a matte containing most of the copper, converting the matte to remove more iron therefrom, blowing the resulting white metal to blister copper, further refining of the copper to produce anode metal and finally electrorefining the latter to pure copper. In the course of smelting the concentrate sulfur dioxide emission problems can be coped with economically by resorting to flash-smelting techniques using pure oxygen to produce relatively low-volume, high-concentration sulfur-dioxide off-gasses. However during the subsequent converting operation sulfur dioxide emission is a more difficult problem to deal with due to the tendency to produce comparatively dilute gas streams. Newer converter designs, and resort to continuous converting practice represent recent steps to deal with the problem.
An important factor in the cost and complexity of pyrometallurgical processing is the handling and treatment of the various slags generated to recover the copper inevitably present therein. Since these have to be treated and fed back to the smelter, they represent a large copper recycle, which is particulaly significant in the case of continuous converting where slags produced are richer in copper than would be the case for batch-converting.
The presence of nickel in the cupriferous concentrate further complicates the smelter operation in that it is generally necessary to ensure a copper to nickel ratio of 100 or more in the blister copper. To achieve this, nickel is removed in the converting cycle and this results in substantial amounts of copper being removed with the slagged nickel, which is generally fed into the nickel recovery circuit. For example, if a concentrate has a copper to nickel ratio of about 20, its treatment by the above route will result in about one quarter of the total copper input being transferred to the nickel recovery circuit with the nickel-containing converter slags. Moreover where cobalt is present overall slag losses lead to poor cobalt recoveries.
More recently an alternative processing route which has been investigated consists in roasting the concentrate and thereafter leaching with water and/or sulfuric acid. Such an approach is described in U.S. Pat. No. 2,783,141 which is directed to the use of fluid-bed apparatus for performing the roast. Variations of the approach include carrying out the roast in more than one stage at increasing temperatures (see U.S. Pat. No. 3,751,240) or at decreasing temperatures (see U.S. Pat. No. 3,883,344). It has also been proposed to use a controlled sulfur dioxide atmosphere during part of the roast (see U.S. Pat. No. 2,817,583) or during the whole roasting stage (see U.S. Pat. No. 3,839,013). A characteristic feature of the processes described in these patents is that the object of the roasting is to ensure sulfation of the copper, and to this end only partial roasting is sought, as opposed to dead-roasting which would leave copper in the form of oxide rather than sulfate. For this reason emphasis is invariably placed on roasting at temperatures below that at which copper sulfate tends to decompose. Thus roasting is carried out below 750.degree. C. and generally at 675.degree. C., 550.degree. C. or even lower temperatures, and such a procedure is said to ensure maximization of the amount of copper which is rendered acid soluble.
An important shortcoming of the above-mentioned prior art roast-leach procedures is that it is difficult to achieve the desired degree of selectivity between copper and other metals, such as nickel, cobalt and iron, which are present in the sulfide treated. In the case of smelted materials, not only is selectivity found to be poor, but indeed the solubilization of copper itself is found to be unacceptably poor. The latter problem has not been confronted by prior workers inasmuch as they have applied the procedure only to ores and concentrates.
It is an object of the invention to provide an improved process for recovering copper selectively from sulfidic materials wherein other metals are intimately mixed with the copper.